This invention relates to hydraulic controlling and driving apparatus adapted to operate movable machine members, such for example as machine tools, assembling, gauging and testing machines, material handling mechanisms or other similar manufacturing equipment.
In accordance with preferred embodiments of the invention, cam driven hydraulic piston pumps are employed to drive hydraulic cylinders which operate the various machine members.
It has previously been proposed to drive machine members by cam driven hydraulic piston pumps such as those disclosed in the patents to Arter U.S. Pat. No. 2,092,721 and Thompson U.S. Pat. Nos. 2,915,855 and 3,071,929. These prior systems lacked dependability and therefore have not had widespread use in industry. Such previous systems worked in a manner similar to the operation of hydraulic tappets used in automotive engines. Such systems employed a single closed fluid column between the cam shaft piston pump and the driven cylinder. Both the pump and the cylinder were single acting. It was necessary to maintain the fluid column in compression and to provide a return force, and for that purpose Arter used a spring. In the Thompson patents, fluid under pressure from a pressurized reservoir was used. Compressed air provided the necessary force to pressurize the fluid in the reservoir.
In both tha Arter and Thompson patents, the cams were required to overcome the returning forces. Thus the cam work force was always larger than the return force. There was also no assurance that the piston would follow the cam if rapid return movement was required. The fluid could cavitate, outside air could leak into the system or air dissolved in the fluid could come out of the solution. The resulting voids in the fluid column caused loss of positive control by the cams. In addition, these single acting closed column systems were limited in their ability to control overrunning loads. The only force available to retard motion initiated by the cam was the limited power of the spring or the compressed air.
The present invention is intended to provide an improved hydraulic controlling and driving apparatus and to overcome the shortcomings in the prior art systems referred to above.
In one or more embodiments of the present invention, a two-line system is used neither one of which constitutes a closed column. Two fluid pressure lines lead to opposite ends of a double-acting motor cylinder. Cam operated pumps are provided to deliver pressure fluid alternately to opposite ends of the cylinder through these lines. Each fluid line is controlled by a valve mechanism.
The valve mechanism in each line includes a check valve which prevents fluid from flowing out of the cylinder back to the pump. On the suction strokes of the pumps, fresh fluid is drawn from an unpressurized reservoir. These check valves prevent the cylinder from motorizing the piston pumps under the influence of a following load.
A second valve in each valve mechanism is a discharge valve. It is caused to open by pilot pressure from the opposite valve mechanism to discharge returning fluid from the cylinder to the reservoir. By discharging fluid returning from the cylinder back to the reservoir and drawing fresh fluid from the reservoir during the suction stroke of the pump, entrapped air has a chance to settle out in the reservoir. This eliminates the air entrapment problem common with the enclosed liquid columns of the prior art devices and prevents the effect of slight leaks in the system from becoming cumulative.
The third valve in each valve mechanism is a suction intake valve to refill the piston pump from the reservoir on each intake stroke. This valve is pilot operated by pressure from the other valve mechanism. Accordingly, the problems of entrapped air and cavitation are reduced.